The BY-CD28/CTLA-4 co-stimulatory pathway has a critical role in regulating T cell activation, and manipulation of this key immunoregulatory pathway has great therapeutic potential. We have cloned a novel murine gene that is a third member of the CD28/CTLA4 gene family and is the murine homologue of the recently described human ICOS (Inducible Co-stimulatory) gene. ICOS is expressed on activated murine T cells and expression as restricted to T cells, similarly to CD28 and CTLA4. In humans, ICOS functions as a receptor for co-stimulatory signals leading to T cell proliferation and production of IL-4 and IL-10, but not IL-2. The relationships of ICOS with the receptors and ligands in the B7-CD28/CTLA-4 pathway need to be define. It is not yet known whether ICOS binds B7 co-stimulators or if ICOS binds to an as yet to be identified counter-receptor in a parallel pathway. It is also not clear when ICOS co-stimulation is needed during immune response. Because ICOS stimulates IL-4 and super-induces IL-10 production, ICOS co-stimulation may have an important role in T cell differentiation and the generation of regulatory T cells. Therefore, manipulation of ICOS signaling may have therapeutic potential. The goal of this research project is to elucidate the role of ICOS co-stimulation in T cell activation, differentiation, and tumor immunity. We have developed a murine ICOS fusion protein that enables us to analyze the function of ICOS on naive and previously activated T cells to characterize the expression and function of ICOS counter-receptor, and to analyze the function of ICOS interactions with the B7/CD28 pathway. In this project, we will characterize how ICOS functions as a receptor for co-stimulator signals from antigen presenting cells and how ICOS regulates T cell proliferation and differentiation. We will examine the role of ICOS co-stimulation in the generation of an effective anti-tumor response in vivo using the EL4 tumor model since the role of IL-4 and IL-10 in suppression of effective tumor immunity has been well defined in this model. ICOS function may deviate the immune response away from a protective cytolytic anti-tumor response and towards a suppressive anti-tumor Th2 response. To understand the role of ICOS in the T cell response and in anti-tumor immunity, I propose to: SPECIFIC AIM 1. To examine the role of ICOS in T cell activation and differentiations. SPECIFIC AIM 2. To examine the expression the expression of ICOS counter-receptor on APC and characterize its molecular structure. SPECIFIC AIM 3. To examine how blocking ICOS signals affect the immune response to murine tumors.